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DAMAGE TOLERANT DESIGN OF FRP SANDWICH STRUCTURES

Objective


A wide collection of damage sources, failures and damage types for typical aeronautical, naval and automotive applications has been catalogued. Low velocity impact damage and debonding of skins were identified as damages with occur most frequently and affect the structural behaviour most severely.

One major part of the project was the development of strength prediction models. One model predicts damage size, and indentation depth as a function of material properties, thickness of components, boundary conditions, and impact energy (Herz static contact law). The second model predicts the failure load of a compression loaded impacted sandwich. The third model is based on the calculation of the strain energy release rate via the change in potential energy of the system due to virtual crack extension. The experimental data obtained showed an acceptable agreement with the analysis results.

Parallel finite element models, generated with general purpose code, have been used for investigations focusing on fracture test specimens. Most of them showed quite good experimental agreement, but the results strongly depend on the used model description and the initial geometric imperfections.

The development of analytical and numerical tools to predict the fatigue life of damages sandwich structures has been the second major part of the programme. Engineering models, based on experimental results of specimens or components, give the possibility to calculate life time prediction by finding a fatigue threshold (load or stress intensity level under which damage does not grow) or by derivation of a synthetic S-N curve combined with linear damage accumulation.

In this project relatively few methods were used for inspection in practice. These methods and other possible methods like sherographic inspection, thermographic inspection and mechanical impedance scans, were evaluated and compared. Damage detection and monitoring during operation was also evaluated.
The damage tolerance behaviour of FRP sandwich structures is a problem of immense practical interest in several industrial fields such as automotive, aerospace, and marine engineering as it effects directly the product safety and economy. Although FRP sandwich construction has been accepted as a basic concept for advanced lightweight structures no generally applicable damage tolerance design concept is available. Due to that high safety factors are used frequently in design inhibiting the efficient use of sandwich capabilities. Therefore, it is proposed to provide an appropriate damage tolerance methodology covering all relevant aspects that have to be considered for design of advanced damage tolerant FRP sandwich structures. The major research tasks will be:

- Development of analytical tools for prediction of residual strength and residual fatigue life of damaged sandwich components.

- Verification of analytical methods by specimen and real component testing.

- Determination of non-destructive inspection procedures applicable on real structures under service conditions and damage detection and monitoring techniques usable during operation.

- Evaluation of new design measures to improve the damage resistance and to limit the effect of doamage on structural efficiency.

All results of the project will be summarized as design guidelines and prepared in a form directly applicable for designers.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

EUROCOPTER DEUTSCHLAND GMBH
Address
Industriestrasse 4
Donauworth
Germany

Participants (8)

Agusta SpA
Italy
Address
Via Giovanni Agusta 520
21017 Cascina Costa Di Samarate (Varese)
Aérospatiale Société Nationale Industrielle SA
France
Address
12 Rue Pasteur
92152 Suresnes
DAIMLER-BENZ AG
Germany
Address
Wilhelm-runge-straße 11
89081 Ulm
DANYARD
Denmark
Address
Niels Jernes Vej 10/Po B.8330
9220 Aalborg Ost.
Defence Evaluation and Research Agency (DERA)
United Kingdom
Address

GU14 6TD Farnborough
Deutsche Forschungsanstalt für Luft- und Raumfahrt eV (DLR)
Germany
Address
Flughafen
38022 Brauschweig
RISO NAT. LABORATORY
Denmark
Address
Po Box 49
4000 Roskilde
Westland Helicopters Ltd
United Kingdom
Address
West Hendford
BA20 2YB Yeovil